It has become common practice in current liquid crystal research to add small concentrations (usually in the range of 0.01 weight % to 5.0 weight %) of nanoparticles of ferroelectric material to a liquid crystal medium. Typical ferroelectric nanoparticle materials may comprise barium titanate (BaTiO3), lithium niobate (LiNbO3), potassium niobate (KNbO3), strontium barium niobate (SBN), potassium sodium strontium barium niobate (KNSBN), spark plasma sintering (SPS), gallium arsenide (GaAs), indium phosphide (InP), and the like. The addition of such materials has been shown to reduce the Freedericksz transition voltage, which measures how much voltage is required to drive a given liquid crystal device, and/or to raise the nematic-isotropic phase transition temperature, thereby increasing the useable temperature range of a liquid crystal-based device. Additional benefits include improved speed of response, higher frequency operation, reduced current leakage, and wider field of view. For example, studies of liquid crystal colloids doped with ferroic nanoparticles have become a topical subject in which the additions of ferroelectric and ferromagnetic nanoparticles have variously been reported to moderate the phase transition temperatures, influence the dielectric anisotropy, affect the electric field induced liquid crystal reorientation (Freedericksz transition), and increase optical diffraction or beam coupling efficiencies. The addition of ferroelectric nanoparticles is therefore of great advantage to many liquid crystal materials. The positive benefits obtained through this addition depend explicitly on the ferroelectric properties of the nanometric material. The production of ferroelectric nanoparticles is, however, rather haphazard and produces materials with uncertain outcomes when added to liquid crystals. Production methods range from chemical sedimentation, to flame photolysis, to mechanical grinding of bulk material. Regardless of the production method, preservation of the ferroelectric property of a strong spontaneous polarization in the prepared nanoparticles is of utmost importance for most applications.
The permanent spontaneous polarization of these materials increases overall liquid crystal sensitivity to externally applied electric fields. The influence of ferroelectric nanoparticles on their environment depends intimately on the net strength of the particle dipole moment arising from the ferroelectric domain spontaneous polarizations. The net dipole moment for any given ferroelectric nanoparticle is maximized when the structure becomes single domain. Unfortunately, common production methods, such as chemical precipitation and spark plasma production, cannot ensure that the resulting nanoparticles have strong ferroelectric dipole moments or that the material is even ferroelectric for smaller size particles, because of the size dependence of the ferroelectric effect. Development of reliable methods to control, or at least to harvest, single domain ferroelectric nanoparticles is therefore of prime interest to many communities. The range of applications that may benefit from a readily available source of single ferroelectric domain nanoparticles is significant. In particular, a need has arisen for harvesting ferroelectric nanoparticles to obtain single domain ferroelectrics at a small nano-scale, such as <10 nm.
Unfortunately, due to this particle size and existing production methods, many nanoparticles have impaired ferroelectric properties compared with the source bulk material. As a result, uniformity and reproducibility in nanoparticle applications are lacking, particularly at smaller sizes, thereby negating the effectiveness of these particles in many applications. The impaired properties of the nanoparticles degrade their performance in many applications and render them useless for applications such as liquid crystal displays. It is, therefore, very desirable to have a method for selectively harvesting those nanoparticles that have the strongest spontaneous polarization and dipole moments from the plethora of non-ideal nanoparticles.